In the fuel cell system which generates electricity through electrochemical reaction of a fuel gas containing hydrogen to an oxidation gas, a widely known technology is a technology of reusing the hydrogen by circulating an anode off-gas on the side of an anode of the fuel cell in order to efficiently utilize the hydrogen contained in the anode off-gas for generating the electricity. In this hydrogen circulation type fuel cell system, it is known that nitrogen transmitting an electrolyte film of the fuel cell from the side of the cathode and impurities contained in the fuel gas are accumulated on the side of the anode electrode of the fuel cell to thereby decrease a hydrogen partial pressure, resulting in a reduction of power generation efficiency of the fuel cell.
Then, such technologies (refer to, e.g., Patent documents 1-4) are disclosed that an electrochemical cell for condensing the impurities by getting the hydrogen contained in the anode off-gas to be selectively transmitted is provided on a circulation route of circulating the anode off-gas for keeping the power generation efficiency as described above, and the impurities in the anode off-gas, which are condensed as a result of the hydrogen transmission, are discharged outside the system. According to these technologies, a timing of discharging the condensed impurities is determined by making use of a value of the voltage applied to the electrochemical cell in order to perform the hydrogen transmission, and the discharge is conducted at a predetermined cycle. The hydrogen partial pressure on the side of the anode electrode can be thus increased by discharging the impurities.